UA does not have a policy of being able to upgrade on other Star airlines with miles, and nor does anyone else.

So just because you bought it on united.com, that doesn't mean they have to upgrade your FRA-TLV segments.

Star Alliance provides reciprocal status benefits such as priority waitlisting, standby, boarding, checkin and extra luggage allowance, along with reciprocal lounge access. They never promised that you will be able to upgrade on other airlines.

Where would you find out the mileage level required to upgrade the FRA-TLV segments anyhow, it's not published the UA MP guide.

I know, it's irritating. But unless you intend to embark on a crusade to make Star change their policy on reciprocal upgrade benefits, I think you might as well try to pick a good seat in Y class for those flights.